HADOOP-19476. Create python3 symlink needed for mvnsite (#7452)

* The mvnsite compilation step needs python3.
  Although we're installing python3 in the build
  environment, the python3 executable is missing.
* Thus, we need to create a symbolic link python3
  pointing to python.exe needed for mvnsite.

dev-support/docker/Dockerfile_windows_10

@@ -147,6 +147,11 @@ RUN setx PATH "%PATH%;C:\Maven\apache-maven-3.8.8\bin"
 RUN setx PATH "%PATH%;C:\CMake\cmake-3.19.0-win64-x64\bin"
 RUN setx PATH "%PATH%;C:\ZStd"
 RUN setx PATH "%PATH%;C:\Program Files\Git\usr\bin"
+RUN setx PATH "%PATH%;C:\Python"
+
+# The mvnsite module runs a bash script and somewhere down in the invocation, it resorts to call
+# /usr/bin/env python3. Thus, we need to create the following symbolic link to satisfy this need.
+RUN powershell New-Item -ItemType SymbolicLink -Path "C:\Python\python3" -Target "C:\Python\python.exe"
 
 # We get strange Javadoc errors without this.
 RUN setx classpath ""